The present invention relates to a process for production of conductive catalyst particles, a process for production of a catalyst electrode capable of gas diffusion, an apparatus for production of conductive catalyst particles, and a vibrating apparatus.
It has been usual practice to produce a catalyst electrode capable of gas diffusion from catalyst particles composed of carbon powder (as a conductive powder) and platinum (as a catalyst) supported thereon, in combination with a water-repellent resin (such as fluorocarbon resin) and an ionic conductor, by forming them into a sheet, for example, as disclosed in Japanese Patent Laid-open No. Hei 5-36418 or applying them onto a carbon sheet.
The electrode thus produced may be used as an electrode for hydrogen decomposition as a constituent of a fuel cell of solid polymer type or the like. In this case, the catalyst (such as platinum) ionizes fuel, giving rise to electrons, which flow through the conductive carbon. The catalyst also ionizes hydrogen, giving rise to protons (H+), which flow into the ionic conducting membrane through the ionic conductor. These actions need interstices for passage of gas, carbon that conducts electricity, an ionic conductor that conducts ions, and a catalytic substance to ionize fuel and oxidant.
A typical way to make carbon powder (as a conductive powder) support platinum (as a catalytic substance) thereon is by dipping carbon powder in a solution containing platinum (in the form of ions), which is followed by reduction and thermal treatment. The processed carbon powder carries platinum fine particles on the surface thereof, for example, as disclosed in Japanese Patent No. 2879649.
The conventional method mentioned above, however, has the disadvantage of requiring the steps for reduction and thermal treatment. With thermal treatment at an inadequately low temperature, it renders platinum poor in crystallinity, which results in mediocre catalytic characteristics.
Moreover, the fact that the carbon powder and the ionic conductor need to be in contact with each other because the catalytic substance like platinum ionizes fuel to give electrons, which flow through the conductive carbon, and also ionizes hydrogen to give protons (H+), which reach the ionic conducting membrane through the ionic conductor, makes it necessary to apply the ionic conductor to the carbon powder on which platinum has been supported. Unfortunately, platinum (as a catalytic substance) functions only at the part which is in contact with gas, and consequently, it becomes unfunctional when it is isolated from gas by the ionic conductor.
There is an alternative method, which consists of coating carbon powder with an ionic conductor and then causing the coated carbon powder to support platinum. This method has the disadvantage of requiring thermal treatment to improve the crystallinity of platinum. However, thermal treatment at a temperature sufficiently high for this purpose deteriorates the ionic conductor which is usually poor in heat resistance.
FIG. 11A is a schematic sectional view showing a conductive catalyst particle (produced by the conventional method) which consists of a carbon particle (conductive powder 1) and platinum particles (catalytic substance 18) supported thereon. Also, FIG. 11B is a schematic sectional view showing a conductive catalyst particle, in which the carbon particle is coated with an ionic conductor 19 and the platinum is supported thereon.
It is obvious from FIG. 11A that the conductive catalyst particles support on the surface thereof platinum in spherical form which has precipitated from the liquid phase. These platinum particles readily separate from the surface of carbon powder. Moreover, production in this manner requires a relatively large amount of platinum. In addition, platinum in spherical form performs its catalytic function only on its surface but does not function inside. Therefore, it has a low catalytic efficiency for its quantity. Another problem is that platinum enters pores in the surface of the carbon powder. (This is not shown.)
Further, causing the carbon powder to support platinum after application of the ionic conductor 19, as shown in FIG. 11B, necessitates thermal treatment to improve the crystallinity of platinum. Unfortunately, the ionic conductor 19 is usually poor in heat resistance and subject to deterioration upon heating at a temperature high enough for the desirable crystallinity of platinum.
A catalytic electrode capable of gas diffusion which efficiently works with a small amount of catalyst is disclosed in Japanese Patent Application No. 2000-293517.
As disclosed, physical vapor deposition, such as sputtering as shown in FIG. 12, makes a catalytic substance adhere to the surface of the conductive powder 1.
However, the production process as disclosed therein consists of depositing the catalytic substance only on the surface of the conductive powder 1 by physical vapor deposition as shown in FIG. 12. Deposition in this manner, it is believed, takes place only on the surface of the conductive powder 1 lying in the uppermost layer in the container 4. Therefore, uniform deposition of the catalytic substance on the entire conductive powder placed in the container encounters difficulties.
A need therefore exists to provide improved processes and apparatuses for producing conductive catalyst particles.